I'm relatively new to electronics - most of my experience is just tinkering with Arduino projects in a more limited sense. I've been working on a project for a week now and have a bit of a (probably basic) quandry.. So I'm trying to convert an SNES controller to be wireless. I've got the SNES Controller side finished - its an ATMega328 + nRF24L01; I'm using the SNESPad library to get input from the controller - I put the state into an int and send it off to the receiver - another ATMega328 and nRF24L01; from there I can bitshift to get the state of each button. That all works perfectly.
What I'm struggling with his how to take the current state and output that back to the SNES. I've looked at the NES/SNES serial protocol, and understand what I need to be doing there, but the only part I can't figure out is how to wire the sucker. The SNES has its 5v vcc line and its ground. I would imagine I would tie its ground to the ATMega's (it's powered by its own source), and not take the 5v. At that point, I should be able to read high/low off of the Clck, latch, and data lines coming from the snes? Or is that not the case?
To sum up the root of the question, I have a powered circuit that I want to interface with another circuit - is sharing ground all I need?